1. Field of the Invention
The present invention relates to a resin composition comprising an ethylene-vinyl alcohol copolymer, which, hereinafter, is also referred to as “EVOH”, a phosphoric acid salt, and a dispersing agent. Multilayer structures having a layer of the resin composition are suitably used as packaging containers to be retorted.
2. Description of the Related Art
Materials of containers to be used for retorting foods such as vegetables and seafood are still dominated by glass, metal and metal foil. Recently, however, rigid or semi-rigid plastic containers have come to be commonly used as containers to be used for retorting other types of food such as soups and pet food. Ethylene-vinyl alcohol copolymer (EVOH) has been adopted as a barrier resin for such plastic containers due to its good processability and excellent gas barrier properties.
One known inherent weakness of EVOH, related to its chemical structure, is the fact that its gas barrier properties are greatly diminished at a relative humidity of 85% or higher. That is to say, water acts as a plasticizer to EVOH, weakens the hydrogen bonding in the amorphous region of EVOH, increases the amount of free volume, and ultimately increases the gas diffusivity through the polymer matrix. When such a phenomenon occurs after typical steam retort treatment where packages are processed at temperature of from 110 to 132° C. for periods of from 15 to 80 minutes, EVOH comes to undergo a term now coined in packaging industry as “retort shock”. During a retort shock, the oxygen transmission rate of the EVOH dramatically increases and for that period of time the food product suffers from a great damage due to oxidation degradation. Packaging and food engineers have studied and designed, by various approaches, packaging containers for food protection with good storage stability. For example, oxygen permeation through a lid is countermeasured by use of a double seamed metal lid or a thick aluminum/polymer lamination.
In the use of EVOH, the oxygen permeability of the container is maintained appropriately by design techniques intended to keep the EVOH as dry as possible. Firstly, the use of relatively thick polypropylene sidewalls was studied. For some applications, sidewalls with a thickness up to 45 mils (1143 μm) have been adopted. Secondly, the use of a relatively thick layer of EVOH was studied and multilayer structures with as much as 20% by weight of EVOH layer were placed in the market. Thirdly, multilayer structures with an adhesive resin layer containing a desiccant to keep the EVOH dry after the retort treatment were placed in the market. The third approach is disclosed, for example, in U.S. Pat. No. 4,407,897 A.
U.S. Pat. No. 4,792,484 A discloses a composition comprising a matrix of an EVOH having dispersed therein a granular drying agent in a particulate state, wherein among the dispersed granular inorganic drying agent, the volume-area average diameter of granules with a long diameter of at least 10 μm is not greater than 30 μm and the weight ratio of saponification product of the ethylene-vinyl acetate copolymer to the inorganic drying agent ranges from 97:3 to 50:50. As a drying agent used herein, a phosphoric acid salt capable of forming a hydrate is disclosed. Multilayer structures having a layer of the resin composition are also disclosed. The multilayer structures are disclosed to be suitable as containers for retort treatment.
It, however, was not easy to disperse desiccant particles uniformly in the matrix of EVOH and therefore agglomerates were readily generated during melt kneading. Generation of such agglomerates will decrease gas barrier properties or will deteriorate appearance of resulting molded articles and also will increase the frequency of screen exchange of extruders.